1. Field of the Invention
The present invention relates to active pixel sensors. More particularly, the present invention relates to full-color detector groups and arrays that use semiconductor material to chromatically filter light vertically and sense multiple wavelength bands at the same location.
2. The Prior Art
MOS active pixel sensors are known in the art. Multiple wavelength active pixel sensors are also known in the art. One type of multiple wavelength active pixel sensor employs red, green, and blue sensors disposed horizontally in a pattern at or near the semiconductor surface. Color overlay filters are employed to produce the color selectivity between the red, green, and blue sensors. Such sensors have the disadvantage of occupying a relatively large area per pixel as these sensors are tiled together in a plane.
Another type of multiple wavelength vertical pixel sensor employs more than one sensor in a vertically-oriented arrangement. An example of an early multiple wavelength vertical pixel sensor for detecting visible and infra-red radiation is disclosed in U.S. Pat. No. 4,238,760 to Carr, in which a first diode in a surface n-type epitaxial region is responsive to visible light and a second buried region in an underlying n-type substrate is responsive to infrared radiation. Contact to the buried photodiode is made using deep diffusion processes similar to diffusion-under-film collector contact diffusion common in bipolar IC processing and for Rcs reduction. The disclosed device has a size of 4 mils square. An alternate embodiment employs V-groove MOS transistors to contact the buried p-type region of the infra-red diode.
The device disclosed in the Carr patent has several shortcomings, the most notable being its large area, rendering it unsuitable for the image sensor density requirements of modern imaging systems. The technology employed for contact formation to the buried infrared sensing diode is not suitable for modern imaging technology or extension to a 3-color sensor.
A particular example of a three-color visible-light prior art vertical pixel sensor group is disclosed in U.S. Pat. No. 5,965,875 to Merrill in which a structure is provided using a triple-well CMOS process wherein the blue, green, and red sensitive PN junctions are disposed at different depths beneath the surface of the semiconductor substrate upon which the imager is fabricated.
This prior three-color sensor group permits fabrication of a dense imaging array because the three colors are sensed over approximately the same area in the image plane. However, this structure has several shortcomings. First, this pixel sensor group uses a reverse-polarity central green-sensitive PN junction, requiring modified circuits or voltage ranges, possibly involving PMOS transistors in addition to the usual NMOS transistors, to sense and read out the green channel. This requirement disadvantageously increases sensor area and complicates support circuits in the array.
Furthermore, an imaging array fabricated using the technology in the Merrill patent requires use of an annular n-well isolation ring within the sensor group pixel area. Such isolation rings consume a large amount of pixel area in the array.
In addition, the junction depths of the three photodiodes are not optimally matched to the absorption depths of the blue, green, and red photons, resulting in inappropriate filter crossover points and poor green filter selectivity.
Finally, the three photodiodes interact with each other. The net result is potential image lag or a requirement for additional control circuitry.
A vertical color filter detector group according to the present invention is formed on a semiconductor substrate and comprises a plurality of detector layers configured by doping and/or biasing to collect photo-generated carriers of a first polarity, preferably negative electrons, separated by additional intervening reference layers configured to collect and conduct away photo-generated carriers of the opposite polarity, preferably positive holes. The detector layers have different spectral sensitivities based on their different depths in the semiconductor substrate, doping levels and biasing conditions. The detector layers are individually connected to active pixel sensor readout circuits. In one example of such a detector group, each detector group includes a blue photodetector n-type layer at the surface of the semiconductor, a green photodetector n-type layer deeper in the semiconductor, and a red photodetector n-type layer deepest in the semiconductor. The blue photodetector at the surface of the semiconductor may optionally have a reference layer only below it, while the red and green photodetectors have reference layers above and below their detector layers. Three sets of active pixel sensor circuitry are coupled to the three detector layers, such that three active pixel sensors are formed using the group of three co-located photodetectors of the vertical color filter detector group.
According to one example, a vertical color filter detector group is formed on a semiconductor substrate and comprises at least six layers of alternating p-type and n-typed doped regions, one of which can be the substrate itself. PN junctions between the layers operate as photodiodes with spectral sensitivities that depend on the absorption depth versus wavelength of light in the semiconductor. Alternate layers, preferably the n-type (but could be p-type) layers, are detector layers to collect photo-generated carriers, while the intervening layers, preferably p-type (but could be n-type), are reference layers and are connected in common to a reference potential referred to as ground. In an embodiment where the detector layers are n-type layers, each detector group includes a blue photodetector n-type layer at the surface of the semiconductor, a green photodetector n-type layer deeper in the semiconductor, and a red photodetector n-type layer deepest in the semiconductor. The blue photodetector layer at the surface of the semiconductor may optionally have a reference layer only below it, while the red and green photodetector layers have reference layers above and below their detector layers. Three sets of active pixel sensor circuitry are coupled to the three detector layers, such that three active pixel sensors are formed using the group of three co-located photodetectors of the vertical color filter detector group. From this disclosure, persons of ordinary skill in the art will recognize that structures are contemplated herein in which other layers, (i.e., intrinsic layers) are disposed between the at least six layers.
According to another aspect of the present invention, a process is disclosed for fabricating the vertical color filter pixel sensor of the present invention.